大兴安岭北段阿里河镇早白垩世高分异花岗岩的确定及其地质意义

Identification and Significance of Early Cretaceous Highly Fractionated Alihe Granites,Northern Great Xing’an Range

大兴安岭广泛出露中生代花岗岩,本文选择大兴安岭北段阿里河镇岩体进行锆石U-Pb年代学和地球化学研究,通过对比大兴安岭北段、中南段和华南等地中生代花岗岩的地球化学特征,探讨岩石成因和产出的大地构造环境。2件花岗岩样品的锆石U-Pb定年结果分别是(134.6±1.5)Ma和(134.4±1.0)Ma,表明岩石的侵入时代应为早白垩世。阿里河镇岩体主要岩石组合为二长花岗岩-正长花岗岩-碱长花岗岩,暗色矿物含量普遍较低,发育晶洞构造。野外调查可见白岗岩和细晶岩出现,且在花岗岩体边部发育伟晶岩脉。该岩体在化学组成上具有高硅(73.63%~77.81%),富碱(Na_(2)O+K_(2)O=7.78%~8.97%),高分异指数DI(91.71~96.80)、过铝质(A/CNK=1.01~1.11)和贫钙、镁的特征,富集大离子亲石元素Rb、Th、U和K,亏损高场强元素Nb、Ta、P和Ti。其微量元素Zr/Hf比值与Zr含量呈正相关,Nb/Ta比值与Ta含量呈负相关,上述变化特征与华南高分异花岗岩类似。综合地质和地球化学特征认为阿里河镇岩体应为高分异I型花岗岩,其动力学背景可能为与蒙古—鄂霍次克洋闭合导致的后碰撞环境有关。

There are voluminous Mesozoic granitic rocks in the Great Xing’an Range.In this study,zircon U-Pb geochronology and geochemistry of the Alihe granites in the northern Great Xing’an Range were analyzed,and were compared with the Mesozoic granites in the northern,middle and southern Great Xing’an Range and South China.The petrogenesis and tectonic setting of the Alihe granites were then discussed.Zircon U-Pb dating on two granite samples yielded Early Cretaceous ages(134.6±1.5 Ma and 134.4±1.0 Ma).The Alihe granites are mainly composed of monzogranite,syenogranite,and alkali-feldspar granites,which contain few mafic mi-nerals and miarolitic cavities.Alaskite and aplite were found in the field investigation route and pegmatite veins were found on the granitic pluton margin.Chemically,these plutons are peraluminous(A/CNK=1.01-1.11)and show high contents of SiO_(2)(73.63%-77.81%),alkalis(Na_(2)O+K_(2)O=7.78%-8.97%),and DI(91.71-96.80),and low Ca and Mg contents.They are also enriched in Rb,Th,U,K,but depleted in Nb,Ta,P,Ti.The varying Zr/Hf and Nb/Ta ratios resemble highly-fractionated granites in South China.Integrating geological and geochemical data,we suggested that the Alihe plutons are highly fractionated I-type granites,which were formed in a post-orogenic extension setting after the Mongolia-Okhotsk Ocean closure.